This invention relates to a method of inhibiting the gross buildup of a cold setting foundry sand mixture upon elements of a mixing machine exposed to the mixture during the process of mixing the mixture, in particular when the mixture has a tendency to adhere to the elements and to set thereupon at ambient temperature to build up a coating on the elements. The invention has particular application to the manufacture of sand-based foundry moulds or cores of the cold-setting type. The normal foundry sand mixture contains sand and a binder; the binder may be a resin and an additional substance such as a catalyst or a resin component can be incorporated in the mixture itself to cause the resin to set. One well-known foundry sand mixture contains a furfural or a urea resin, which is caused to set by an acid catalyst or hardener; a further acid-hardening resin is a phenolic resin. An example of another type of resin is a polyurethane resin, which is normally hardened by the addition of a diisocyanate, which is somewhat alkaline.
The normal foundry sand mixtures are tacky, usually because of the resin precursor in the mixture, and the mixture adheres to every part of the interior of the mixing machine.
The invention particularly relates to mixing machines in which at least one moveable mixing element passes close to the walls of the mixing machine and moves at a sufficiently high speed for centrifugal force to prevent gross build-up of the onset mixture on the moveable element. In one particular mixing machine of this type, the mixing element rotates at a speed of about 500 r.p.m., and there is a clearance of about 1/8 inch between the periphery of the mixing element and the internal walls of the mixing machine. The mixing machine may have a main trough 6 feet long and about 6 inches diameter.
If nothing is done about the adherence of the mixture to the interior of the mixing machine, the adhered layer sets during the course of mixing a number of batches. Only a thin layer initially remains on the mixing element because centrifugal force throws off any larger lumps of material, but once the thin layer has set, a further thin layer can adhere and in turn set, leading to gross build-up of hardened material on the mixing element. In the case of the walls of the mixing machine, the mixing element prevents any initial build-up greater than the clearance between the mixing element and the walls; however, as the mixture sets, it initially shrinks, allowing slight further build-up, and then it warms up and expands, such expansion being required for good formation of the moulds; due to this expansion, the mixing element fouls the material and the mixing machine bangs and shakes, causing extreme wear of the mixing element. In addition to the unacceptable banging and shaking and extreme wear, the actual efficiency of the mixing machine is reduced, particularly due to the build-up on the mixing element, and furthermore set material breaks off and can be incorporated in the foundry mould or core and later require the mould or core to be scrapped.
In general, the only way of cleaning the mixing machine was to withdraw the mixing element and chip off all the set material, which was done in normal practice at least twice a day. This causes great loss of machine time. An alternative method of cleaning the machine was to remove the adhered mixture before it had set using high pressure air jets, but I believe that this way always done using a lance held in the hand, and this cleaning procedure took a significant amount of time and was thus relatively expensive.
In a different field, that of mixing foodstuffs, U.S. Pat. No. 3,138,167 in the name of Chester Donald Fisher discloses a mixing machine in which pressure gas or liquid is directed to all parts of the interior of the mixing machine after mixing each batch, in order to remove every trace of the previous batch. However, this also can be time-consuming and I have discovered that such a procedure is not necessary to avoid the problems given in mixing cold-setting foundry sand mixtures.